Coagulation of Blood

When you cut your finger, the bleeding does not go on indefinitely; after a short while the blood stops flowing on its own. This happens because blood can coagulate, or clot, in response to any injury or trauma. Clotting is a protective mechanism that keeps the body from losing too much blood through a wound.

The clot itself

At the site of a cut, a dark reddish-brown mass gradually builds up over the wound. This is the clot (also called a coagulum). Structurally, it is a mesh of thread-like fibres called fibrin, and caught within this net are the dead and damaged formed elements of the blood. The tangled fibrin threads plus the trapped cells together seal the injury.

How the clot forms

The formation of a clot is not a single step but a chain of conversions, each turning an inactive substance into an active one:

• Fibrin threads are produced when inactive fibrinogen, present in the plasma, is converted into fibrin.
• This conversion is carried out by an enzyme called thrombin.
• Thrombin itself is made from another inactive plasma substance called prothrombin.
• The conversion of prothrombin to thrombin needs an enzyme complex called thrombokinase.

Thrombokinase does not appear ready-made. It is built up through a series of connected enzyme reactions — a cascade — that involves a number of factors that normally circulate in the plasma in an inactive form.

What sets the process off …